Digital subscriber line (DSL) technologies provide a large bandwidth for digital communications over existing subscriber lines (e.g., copper pairs). To transmit data signals, many current DSL systems, including asymmetric DSL 2 (ADSL2), ADSL2+, very high speed DSL (VDSL), and very high speed DSL 2 (VDSL2), and other DSL systems, including Fast Access to Subscriber Terminals (G.fast), a consented standard, may employ discrete multi-tone (DMT) modulation. Systems that perform duplex transmission using frequency division duplex (FDD), such as ADSL2 and VDSL2, separate downstream signals from the upstream signals by exchanging the signals using different frequency bands. Alternatively, systems that perform duplex transmission using time-division duplex (TDD) may use separate time intervals for upstream and downstream transmission. During upstream transmission from a remote side modem, for example, there may be no downstream transmission in a G.fast DSL system from a corresponding modem at a central office (CO), a fiber to the curbside cabinet (FTTC), or distribution point unit (DPU).
When transmitting data over the subscriber lines, crosstalk interference can occur between the transmitted signals over adjacent lines, for example in a same or nearby bundle of lines. Crosstalk, including near-end crosstalk (NEXT) and far-end crosstalk (FEXT), may limit the performance of various DSL systems, such as those defined by standards including ADSL2, VDSL, VDSL2, and G.fast. Typically, FEXT levels increase and become more problematic as the high frequency band edge increases for DSL systems. For example, a VDSL2 system may operate at bandwidth frequencies ranging from about 17 to 30 megahertz (MHz), while a G.fast DSL system may operate at bandwidth frequencies ranging from about 100 MHz and higher. As such, the FEXT levels within a G.fast DSL system may be relatively higher (e.g. FEXT levels may be as strong as the data signal) than a VDSL2 system.
Vectoring techniques may be used to cancel FEXT amongst subscriber lines within a vectored group in the downstream and upstream directions for DSL systems (e.g. VDSL2 and G.fast). Vectoring cancels crosstalk by coordinating and managing a group of subscriber line signals in order to reduce crosstalk levels. Vectoring is described in more detail in the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) G.993.5, entitled “Self-FEXT cancellation (vectoring) for use with VDSL2 transceivers,” updated April 2010, which is hereby incorporated by reference as if reproduced in its entirety. In vectored DSL systems, in order to implement downstream and upstream vectoring, the vectored DSL systems may implement symbol alignment. Specifically, downstream symbols transmitted by transceivers at the operator end (TU-Os) of a vectored group may be aligned between themselves at an operator side interface (U-O referenced point) and upstream symbols transmitted by transceivers at the customer end (TU-Rs) of a vectored group may be aligned between themselves at the U-O reference point. Symbol alignment in the downstream direction may be achieved by transmitting DMT symbols at the same time on all of the subscriber lines in the vectored group because the TU-Os are typically co-located, clock synchronized, and more likely within the same DSL access multiplexer (DSLAM) equipment. Unfortunately, in comparison to the downstream direction, symbol alignment in the upstream direction may be relatively more difficult because the TU-Rs are generally situated at different locations.